Devices utilizing the giant magnetoresistance (GMR) effect have utility as magnetic sensors, especially as read sensors in recording heads used in magnetic disc storage systems. The GMR effect is observed in thin, electrically conductive multi-layer systems having multiple magnetic layers. One sensor type that utilizes the GMR effect is the GMR multi-layer. The GMR multi-layer typically comprises a series of bi-layers, each of which includes a thin sheet of a ferromagnetic material and a thin sheet of a non-magnetic material. The bi-layers are stacked to form a multi-layer device. The multi-layer device is usually mounted in the read head so that the layers are perpendicular to the plane of the disc.
In operation, a sense current is caused to flow through the read head and therefore through the sensor. Magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the layers, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
The output voltage produced by the sensor is affected by various characteristics of the sensor. The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane or CIP configuration. Alternatively, the sense current can flow through the sensor in a direction that is perpendicular to the planes of the layers or stacked strips that comprise the sensor. This configuration is known as a current-perpendicular-to-plane or CPP configuration.
The CPP sensor is interesting because of its potentially larger giant magnetoresistance (GMR) or change in resistance when a magnetic field is applied. The CPP sensor is therefore capable of producing a higher output voltage than the CIP sensor, which results in a more precise and sensitive read head. The larger change in resistance comes about because all of the current needs to pass through every ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) series of interfaces. Because every film and interface leads to additional resistance, it is desirable to have all of the layers and interfaces contribute to the overall change in resistance ΔR of the device.
GMR devices having the described CPP configuration have the potential to be used as read back sensors in data storage systems operating at areal densities on the order of about 1 Tbit/in2. CPP GMR devices offer the prospect of relatively large magnetoresistance ratios, ΔR/R. However, it has been observed that CPP read back sensors produce a significant amount of noise that limits the practical application of CPP GMR devices in recording heads. It has been determined that a major source of this noise results from a phenomenon called spin momentum transfer, which generally refers to the exchange of spin angular momentum between conduction electrons and the magnetic moment of a ferromagnet. This spin momentum transfer effect leads to torques that act on the ferromagnetic layers within a CPP device, consequently leading to unintended magnetization dynamics or noise. This effect will occur between any two adjacent ferromagnetic layers such that every layer within a CPP GMR multi-layer device will fluctuate and contribute to device noise. Therefore, CPP devices that fail to account for spin momentum transfer will inevitably be noisy.
Accordingly, there is a need for an improved CPP configuration that overcomes the limitations, disadvantages, or shortcomings of known CPP configurations.